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Do varroa mites actually carry viruses on them ?
Is it true that they are a major source of certain viruses.?
Whilst it is widely held that varroa is a vector for viruses, I took this to mean that the weakening of the bees caused by varroa
left them susceptible to viruses.

It would strike me as odd, that a parasite (which varroa is in relation to
Apis Cerana), would damage its host in such a way.

Varroa jacobsoni (Varroa mites) are suspected as vectors of several viruses in Apis mellifera (the common honeybee).

Vectors normally do carry and thus transmit the disease to another species. Vectors are often "dead end hosts" of the disease themselves - meaning it does not notably harm them but they are carriers.

In Apis mellifera it is quite possible to induce a number of diseases simply by adding different stress factors. Any type of parasite places stress upon its host. High parasite loads reduce offspring birth weights (in bees this weight can never be regained), decrease longevity, and a variety of other negative things.

Let me give you an example. At one time MSU was studying Chalkbrood prevalence in pollination colonies. They found that in their MSU test yard if they simply picked up the colonies, set them on a pickup at night, and drove around the block (4 miles) then set the colonies right back on their original hive stands - that was sufficient stress to induce chalkbrood in most colonies. Colonies which had shown NO chalkbrood previous to the new and additional stress.